The present invention relates to a solution of a polyurethane in a high molecular weight polyether having terminal hydroxyl groups. Such a solution may be used as a modified polyol in the production of polyurethane plastics.
Dispersions of polyisocyanate polyaddition products in polyols are known. For example, German Auslegeschrift No. 1,168,075 teaches a process in which diisocyanates are reacted in situ with difunctional, diprimary alcohols in a polyether dispersing medium. The polyether used in the disclosed process must have a molecular weight of from 500 to 3,000 and at least two secondary hydroxyl groups in the molecule. Since primary alcohols react more rapidly with isocyanates then do secondary alcohols, only the bifunctional, primary alcohols take part in the polyaddition reaction. The higher molecular weight polyether with secondary hydroxyl groups, therefore, serves as a virtually inert reaction medium. The product polyurethane is in the form of finely dispersed particles and does not deposit sediment, even after a comparatively long standing time.
German Offenlegungsschriften Nos. 2,550,796 and 2,550,797 teach stable dispersions of polyisocyanate polyaddition products in compounds having hydroxyl groups as the dispersing agent. These dispersions are obtained by an in situ reaction of polyisocyanates with a variety of active hydrogen containing compounds in the presence of specified quantities of water. Among the disclosed reactions is the reaction of diisocyanates with diprimary diols in polyethers having secondary hydroxyl groups to form dispersions of finely divided polyurethane particles.
When dispersions of organic fillers in higher molecular weight hydroxyl compounds are used as starting materials for the production of polyurethane plastics, some of the properties of the plastic are greatly improved. For example, increased hardness and improved heat resistance may be achieved without detrimentally affecting the other properties of the plastic, but these improvements are limited in the same manner as improvements due to use of other "active fillers." The properties of the polyurethane platics produced from various dispersions of different organic fillers in higher molecular weight hydroxyl compounds are (with certain limitations) independent of the chemical nature of the filler where the filler used in each dispersion has a particle size and concentration comparable to that of the other fillers in the other dispersions. Thus, use of acrylonitrile-styrene-copolymers, polyhydrazo-dicarbonamides, polyureas, ureaformaldehyde resins and high melting polyesters as fillers may produce results which are substantially the same.
A dispersed filler would have a greater effect upon the properties of the product plastic if the higher molecular weight additives mentioned could be incorporated homogeneously into the polyurethane. However, such homogeneous incorporation is generally unattainable because these additives are insoluble in the starting materials used in preparing polyurethanes.
Solutions of polyisocyanate polyaddition compounds in polyols are also known to those in the art. In German Offenlegungsschrift No. 2,638,759, e.g., such polyisocyanate polyaddition solutions having a solids content of from 5 to 70% by weight are described. These solutions are produced by reacting diisocyanates with compounds which contain active hydrogen atoms (e.g., diprimary diols) in a reaction medium which is a polyhydric alcohol having a molecular weight of between 62 and 450 (generally between 62 and 200). These polyisocyanate polyaddition solutions were also prepared by dissolving the pulverized polyaddition compounds (which had been previously prepared) in the polyhydric alcohols described above.
While such polyisocyanate polyaddition solutions may be used as starting materials in the production of polyurethane plastics, they are unsuitable for preparing flexible elastomers, flexible coatings and flexible foamed plastics. Flexible products cannot be made with these solutions because addition of the appropriate amount of polyaddition compound results in the presence of comparatively large quantities of the solvent low molecular weight polyhydric alcohol. Such quantities of the solvent alcohol make the product polyurethane plastics undesirably brittle.
It would be particularly advantageous, therefore, to have solution of a polyurethane in a polyol which could be used in making flexible elastomers, coatings and foamed plastics.